Hands on: Cel-Fi mobile repeater

If your home is a mobile blackspot then a Cel-Fi mobile repeater might help you stay connected.

If you've got solid mobile phone coverage throughout your home then you probably take it for granted, but not everyone does. Mobile blackspots don't just affect people who live in the middle of nowhere, blackspots are also scattered across the suburbs depending on your telco and choice of handset. Reception can even vary from room to room as you move around the house.

If you can get a sniff of mobile coverage outside, but it drops out when you walk in the front door, then a Cel-Fi mobile repeater might be able to come to your rescue.

The Cel-Fi consists of two parts. The receiver unit (above on the left) sits next to a window to pick up the weak mobile signal from outside. The Window unit then relays this signal to the smaller Coverage unit, which you can place in a central part of your house for best indoor coverage.

The Cel-Fi can turn one bar of outdoor coverage into five bars of indoor coverage, but it's not a miracle worker. If the Window unit can't get a sniff of the mobile network then you're out of luck. The Cel-Fi starts at $748, but you can pay extra for an external antenna to improve the Window unit's reception.

Is the law on my side?

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Mobile repeaters have been around for a long time, but telcos don't like to talk about them – partly because they don't like to admit their networks aren't perfect and partly because unauthorised mobile repeaters can cause interference for your neighbours. It's illegal to use an unauthorised mobile repeater but the Cel-Fi is approved for use in Australia and has the blessing of the local telcos – but they give the Cel-Fi little publicity.

A few years ago I tested the Optus 3G Home Zone, designed to do a similar job, but the Cel-Fi has several major advantages. Firstly, the Optus 3G Home Zone is an all-in-one unit. Secondly, you can buy a Cel-Fi to use with any Australian mobile telco, you just need to specify which one when you order the unit.

Thirdly, the Cel-Fi isn't reliant on internet access so you don't need to mess around with port forwarding and other messy configuration issues. Finally, you don't need to register the Cel-Fi to a specific address which means, if you really need to, you can take it with you when you travel. You need to check the fine print with your telco, as the FAQs reveal that Optus locks the unit to the mobile towers in a specific area after 32 hours of use – similar to the 3G Home Zone.

What's the frequency?

I tested out a Telstra-compatible Cel-Fi which connects to the 850 MHz band, but you'll find 900/2100 MHz units for other telcos. There's no support for the new 700 MHz band, which is designed for better rural coverage and indoor reception. It's possible that when 700 MHz coverage reaches your area the indoor reception might improve, but only if you have a 700 MHz-compatible handset such as the;

Apple iPhone 6, 6 Plus

Samsung Galaxy S5

LG G3,

HTC One M8

Sony Xperia Z3, Z3 compact

Lumia 830

Unfortunately some 700 MHz handsets like the Apple iPhone 5S/5C, Xperia Z2 and Lumia 930 aren't compatible with Australia's 700 MHz networks so you won't see an improvement from the 700 MHz rollout.

Up and running

The Cel-Fi is simple to set up. Firstly, walk around your home with your phone to find the spot with the best coverage – perhaps next to an upstairs window. Plug in the Window unit, wait for it to power up and check the front display to see if it has at least one bar of coverage. Experiment with different positions to find the strongest signal.

Next find a central place to plug in the Coverage unit, up to 20 metres away from the Window unit. Wait for it to power up and then check the front display to see if see if it's too close to the Window unit or too far away (they use a 5GHz link). You want to find the sweet spot in the middle where the signal strength is around 8.

That's really all there is to it. Once it's set up the Cel-Fi can improve both your voice reception and your mobile broadband reliability. There's no hard limit on the number of users, it depends on local conditions, but the fine print says it's "expected to support the needs of a small office/family residence".

There's no light on the Coverage unit to indicate when it's talking to a handset. You'll know when your handset has switched across because you'll go from a weak 3G or 4G signal to a strong 3G signal. If you've already got a decent 4G signal from a nearby tower then falling back to 3G might see your mobile data speeds drop, but if this is the case you probably don't need the Cel-Fi.

If you switch your handset on and off it will usually link to the Cel-Fi first, but if it can see a reasonable 4G signal from the nearest mobile tower it will automatically switch across. In my tests, my iPhone 5 happily maintained a call as I walked out the front door and roamed off the Cel-Fi onto Telstra's mobile 3G network – after I ended the call it switched across to 4G. The same applies coming back home, the phone happily roaming on and off the Cel-Fi during a voice call without disruption.

So what's the verdict?

Achieving decent mobile coverage in a blackspot is a bit of a dark art and your mileage may vary, but the Cel-Fi clearly helps. The price tag is a little steep, especially when you're paying to fix your telco's shortcomings, but it's money that some people might be prepared to pay for trouble-free home reception.

It's possible that the new 700 MHz mobile networks might come to your rescue, but even then the Cel-Fi could still work out to be more cost effective if it means you're not forced to upgrade several phones to tap into 700 MHz coverage.